Spray a 9×13 baking dish with nonstick cooking spray, or you can lightly coat it with butter.
Combine the ham, Parmesan cheese, onions, and croissants in a medium mixing bowl. Arrange in prepared baking dish.
Whisk together milk, cream, and eggs and pour over the croissant mixture. Cover and chill overnight.
Preheat oven to 350° F. Uncover the casserole and sprinkle with the Cheddar cheese.
Bake until golden brown and cooked throughout, about 45 minutes. Remove from the oven and let stand for 5 minutes before serving.